1. Technical Field
The present invention relates to display devices and their controllers, and more particularly, to black display in display devices for computers, such as CRT display devices and liquid crystal display devices.
2. Description of the Related Art
Color display on the computer display device relies theoretically on an additive mixture process of three primary colors red, blue and green (referred to as RGB). More specifically, changing the luminescence intensity of each of three color elements of RGB for the area where a predetermined color of the display device is desired, ensures that it is seen as the predetermined color by the eye. By adjusting the luminescence intensity of each of three color elements of RGB, in principle, all colors within a triangle formed by connecting three points of the RGB primary colors in the chromaticity diagram can be displayed. Therefore, displaying a black image on the display device, for example, can basically be accomplished by adjusting the luminescence intensity of each of three RGB picture elements. Displaying black images, however, can be done simply by not illuminating the display screen. Thus black images have been displayed by preventing illuminance of all three color elements of RGB for the area of the display screen where the images are desired to be displayed.
The display device for computers, such as personal computers, has increased in color information to be handled, ranging from single colors for character display, up to eight colors for business graphics, up to 256 colors for simplified natural color display, to over several thousands colors for multimedia. Along with this trend, the interface between the display device and the display device controller in a computer has changed its display method from that of digital to analog RGB, which is able to display finer colors.
The interface between the computer display device and its controller is configured as shown in FIG. 1, for example, by connecting display device controller 2, wherein video data to be displayed is input, provided within personal computer 1, and display device 3 such as a CRT display device and liquid crystal display by means of five signal lines: three RGB signal lines, vertical (V) and horizontal (H) synchronous signal lines.
Generally, for analog RGB process, three RGB analog signals having an amplitude of 0 V to 0.7 V are transferred from the controller to the display device via color signal lines. For each one of RGB analog signals, 0 V corresponds to black and 0.7 V corresponds to white, and the color intensity of each primary RGB color is adjusted in a range from 0 V to 0.7 V. For the CRT display device, for example, each analog RGB signal being transmitted is amplified to a signal having a predetermined voltage ranging from a black level potential to a white level potential and applied to the CRT cathode, thus each picture element of RGB for the portion, wherein a predetermined color is desired to be displayed, of the CRT being illuminated at a predetermined intensity.
The display device has generally a manual control for adjusting brightness, allowing the user to change the brightness of the display screen. Referring to FIG. 2, to make the screen of the CRT display brighter, for example by moving the manual control, the entire color signal being amplified within the display device is shifted from the black level potential to white level potential and the voltage applied to the CRT cathode is increased, thus the whole screen is made brighter. In this case, however, a portion of signal which should be at the black level potential for displaying a black image on the screen also shifts toward the white level potential. Consequently, the entire display screen becomes "whitish"-looking. Three RGB picture elements for the portion of the screen where black is desired to be displayed are illuminated, and thus, even the portion which is desired to remain black also becomes whitish.
In addition, for the multimedia display device, for example, clearer color pictures are desired, and particularly black plays a very important role in more clearly displaying chromatic colors on the screen. Therefore, it is necessary for displaying clearer color images to have black manifest itself, i.e. to ensure that black images are displayed in an area where they must be displayed on the display device.
In such a situation, conventional computer display devices have given little or no special consideration to black image display. No prior art discloses a novel means for taking out a signal which should display a black image and using it to force the display device to display the black image.